During his life, Shong Lue produced four versions of the script. The first stage consisted largely of identification of the sounds; each vowel+tone had a different sign, as did each consonant. In the second stage the script was made more economical; tones were divided into two groups, marked by diacritics. The 60 consonant symbols were reduced to 20, and 2 diacritics introduced to modify these and represent the full consonant inventory. A further refinement in the third stage systemized the associations between vowel qualities and symbols, and by the fourth and final stage each vowel quality was associated with only one symbol, which could be modified by a greater number of diacritics, each diacritic representing one tone.
Martha Ratliff, 'The Pahawh Hmong Script', in The World's Writing Systems, ed. by Daniels & Bright (Oxford: Oxford University Press, 1996), pp. 619-120.